The present invention relates to high voltage low inductance resistors and is particularly related to a resistor used to regulate transient current flow caused by electrical discharge within high voltage electrical equipment. The present invention finds particular application in conjunction with high voltage vacuum tubes, particularly x-ray tubes, and will be described with respect thereto.
Conventional diagnostic use of x-radiation includes radiography, in which a still shadow image of the patient is produced on x-ray film, fluoroscopy, in which a visible real time shadow light image is produced by low intensity x-rays impinging on a fluorescent screen after passing through the patient, and computed tomography (CT) in which complete patient images are digitally constructed from x-rays produced by a high powered x-ray tube rotated about a patient""s body.
Typically, an x-ray tube includes an evacuated envelope made of metal, glass, ceramic materials or combinations thereof which is supported within an x-ray tube housing. The x-ray tube housing provides electrical connections to the envelope and is filled with a fluid such as oil to aid in cooling components housed within the envelope. The envelope and the x-ray tube housing each include an x-ray transmissive window aligned with one another such that x-rays produced within the envelope may be directed to a patient or subject under examination. In order to produce x-rays, the envelope houses a cathode assembly and an anode assembly.
The cathode assembly includes a cathode filament through which a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e. thermionic emission occurs. A high potential, on the order of 100-200 kV, is applied between the cathode assembly and the anode assembly. This potential causes the electrons to flow from the cathode assembly to the anode assembly through the evacuated region in the interior of the evacuated envelope. A cathode focusing cup housing the cathode filament focuses the electrons onto a small area or focal spot on a target of the anode assembly.
The electron beam impinges the target with sufficient energy that x-rays are generated. A portion of the x-rays generated pass through the x-ray transmissive windows of the envelope and x-ray tube housing to a beam limiting device, or collimator, attached to the x-ray tube housing. The beam limiting device regulates the size and shape of the x-ray beam directed toward a patient or subject under examination thereby allowing images to be constructed.
In order to distribute the thermal loading created during the production of x-rays a rotating anode assembly configuration has been adopted for many applications. In this configuration, the anode assembly is rotated about an axis such that the electron beam focused on a focal spot of the target impinges on a continuously rotating circular path about a peripheral edge of the target. Each portion along the circular path becomes heated to a very high temperature during the generation of x-rays and is cooled as it is rotated before returning to be struck again by the electron beam.
Typically, the anode assembly is mounted to a rotor which is rotated by an induction motor. The anode assembly and rotor are part of a rotating assembly which is supported by a bearing assembly.
During operation, the x-ray tube presents a high impedance of several hundred thousand ohms to the voltage applied between the anode assembly and cathode. This results in a relatively small current flow through the vacuum space between the anode assembly and cathode assembly. Under normal operating conditions, the power source is capable of regulating the current flow between the anode and cathode. Despite the regulation by the power source and the electrical isolation of the anode and cathode, when two elements with such a large difference in potential are placed proximate to each other, there is a tendency to arc. An arc is an undesired surge of electrical current between two elements which are at a different electrical potential.
In an x-ray tube, arcing can occur through residual gas molecules present within the evacuated envelope of the x-ray tube. As an x-ray tube ages, the tendency to arc often increases due to such factors as degradation of the vacuum within the tube resulting in increased gas pressure. The increased gas pressure within the evacuated envelope is due to the existence of additional undesired gas molecules. For example, gas molecules may migrate through the envelope, evolve from the materials within the envelope or are released as a result of damage to the components due to arcing. Consequently, the mean free path between gas molecules is reduced such that a chain reaction is more likely to occur when the gas molecules in the vacuum envelope are ionized by the high electric fields generated during normal tube operation. This chain reaction is called avalanche and is a form of arcing.
Arcing typically occurs in an area of the x-ray tube having the highest electric field strength. As such, arcing in an x-ray tube will commonly occur in the general region where the cathode is supplying the anode with electrons for the production of x-ray emissions. In addition, the structural imperfections of the electrodes contribute to the location where arcing occurs. This is because there are intense electric field gradients caused by contamination, sharp corners or rough edges on the surfaces of the electrodes. In particular, fields are higher where there are surface imperfections on the anode disk.
One consequence of arcing is the radiation and conductance of intense electrical noise on the high voltage electronic components. These noise emissions can cause failure of semiconductor devices in the system circuitry.
Another effect of arcing is the sputtering of metal from the cathode produced during arcing often lands on the internal surface of the glass envelope in proximity to the cathode. The existence of the metal deposits on the glass envelope can deleteriously effect x-ray tube performance for several reasons. First, as arcing occurs from time to time, sputtered metal deposits will continue to grow. As the sputtered metal deposits on the glass envelope gets too thick, an electrical charge may accumulate sufficient to damage the glass envelope thereby rendering the tube nonfunctional. Secondly, sputtered metal deposits on the glass envelope will often attract arcing between the deposits and the cathode. The surges of electrical current produced during arcing can damage the glass envelope, again rendering the tube nonfunctional.
When the x-ray tube arcs, a current on the order of hundreds of amperes can flow between the cathode and the anode. Once an x-ray tube starts to arc, an avalanche type effect may occur sputtering metal and the metal atoms as well as ionizing the contaminants in the vacuum. These events further contribute to yet more frequent arcing. In addition, arcing in an x-ray tube used in a Computed Tomography (CT) imaging system contaminates the x-ray signal collected at the detectors and affects proper image reconstruction. This may result in an un-usable set of data requiring another CT scan of the patient.
As mentioned above, arcing can shorten the useable service life of the x-ray tube. Given the considerable cost of an x-ray tube and the associated service costs for replacement, it is desirable to extend the service life of the x-ray tube.
One known method to extend service life and reduce arcing involves providing getter material inside the glass envelope to help maintain the evacuated state. The getter material binds gases on its surface and absorbs such gases to maintain the vacuum state in the x-ray tube. The process of removing residual gases from an evacuated area by binding and absorbing is known as pumping. By using getter material to maintain a vacuum state, arcing is reduced since there is a reduction in the number of gas molecules through which large current surges may flow. Unfortunately, as the x-ray tube ages the effectiveness of the getter material in pumping also diminishes. As a result, arcing tends to become more frequent as the getter is used and the tube ages.
Information relevant to other attempts to address the problem of transient current surges during arcing can be found in U.S. Pat. Nos.: 5,229,743; 5,107,187; 5,132,999 and 5,008,912. However, each of these references suffers from one or more of the following disadvantages: (i) the transient control apparatus is too large to be located near enough to the anode terminal of the x-ray tube, (ii) mechanical failure of the device from limiting the damaging current flow, (iii) difficult and costly to manufacture (iv) inconsistent electrical characteristics such as inductance, voltage drop etc. (v) low reliability and (vi) lower surge load capacity.
Referring to FIGS. 1, 2 and 3, a prior art low inductance resistor 80 is shown that is used in an x-ray system 20 as a resistor 76. The resistor 80 is circular and has a diameter slightly larger than the outside diameter of a socket member (not shown) for receiving and electrically connecting an anode end 81 of an x-ray tube 24 to a power supply 22. The resistor 80 includes a conductive cylindrical high voltage terminal 82 having a threaded inner surface for receiving a mounting bolt (not shown). The mounting bolt secures a terminal (not shown) connected to a high voltage conductor 74 to electrically connect the resistor 80 to the power supply 22. An electrically conductive annular hub 86 is located at the center of the resistor 80. Both the high voltage terminal 82 and the annular hub 86 serve as electrical terminals to electrically connect the resistor 80 between the conductor 74 and an anode end 81 of the x-ray tube 24. The annular hub 86 includes a bore 88 for allowing passage of a threaded bolt (not shown) that is threadably received in a bore (not shown) in the anode end 81 of the x-ray tube 24 to secure the resistor and complete the electrical connection of the resistor 80 in the circuit. A body 90 of the resistor 80 is formed from an electrically non-conductive resin and is hardened with a hardener and vacuum molded. A barrier 98 of non-conductive body material is located between the terminal 82 and hub 86.
The terminal 82 is electrically connected to a ring 100 of conductive material having a diameter slightly less the diameter of the resistor 80. The conductive ring 100 is split at one point and the ends are suitably attached to the conductive terminal 82 such that an appropriate electrical connection is completed for use at the anticipated operating and arcing conditions experienced by the x-ray tube. The electrical resistance of the resistor 80 is provided by two spirally wound coils of resistance wire 94 and 96. The wires 94, 96 are electrically connected at one end to the hub 88 at a point 102 and at the other end to conductive ring 100 at a point 104. The two spirals of resistance wire 94, 96 are counter wound and laid out in parallel planes within the resistor 80. When energized, the current in each wound spiral of wire flows in the respective directions of arrows 106 and 108. Each spiral consists of approximately of 60 turns of wire. The resistance wire coils 94, 96 are connected in electrical parallel between the ring 100 and the hub 86. Referring to FIG. 3, the wire coils 94, 96 are spaced apart and electrically isolated from one another with a layer 110 of the electrically non-conductive resin.
In prior art multi-planar resistor devices, the distances between the two spiral wound resistive elements can vary. This can result in varying the distances between the magnetic fields generated in each of the spiral wound resistor wires 94, 96 as well as the uniformity of the resulting magnetic fields across the planar surfaces of the resistor. In addition, interaction between the magnetic fields of the spiral wound wire coils during higher current and fault conditions generate forces on the coils and other components of the resistor that result in mechanical and/or electrical failure of the resistor. Irregularities in the magnetic fields due to variation in coil spacing may cause localized in-homogeneity resulting in failure. Such a failure in an operating x-ray system requires an expensive repair before the system is returned to specified operating parameters.
For the foregoing reasons, there is a need for an apparatus for the reduction of arcing and associated current surges in x-ray tubes that is more easily manufactured, has more consistent electrical characteristics and has improved durability.
The present invention is directed to a low inductance resistor that satisfies the needs described above. An apparatus in accordance with one embodiment of the present invention includes a resistor body that has a perimeter and a center. A first terminal is located away from the center of the resistor near the perimeter. A serpentine resistance element has a first end and a first resistance segment which begins at the first end. The first resistance segment extends in a first direction generally around the perimeter of the body, e.g. in a clockwise direction. The resistance element includes a generally xe2x80x9cUxe2x80x9d shaped apex having an input side and an output side. The first resistance segment transitions into the input side. The resistance element includes a second resistance segment that exits the apex from the output side in a second direction generally opposite that of the first resistance segment, e.g. counterclockwise. The second resistance segment is located adjacent to, and spaced apart from, the first resistance segment. The pattern formed by the first and second resistance segments provides a concentric serpentine pattern located in a single plane. The resistance element includes second end located approximately at the center of the resistor element. A conducive ring circumscribes the serpentine resistance element. The ring is electrically connected to each of the first terminal and the first end of the serpentine resistance element. A second terminal is located at the center of the resistor and is electrically connected to the second end of the serpentine resistance element.
In accordance with a more limited aspect of the present invention, the electric current in the adjacent first and second resistance segments flows in a generally opposite direction. In another aspect of the invention, the second resistance segment is shorter in length than the first.
In accordance with another limited aspect of the invention, the resistor element includes a plurality of additional adjacent concentric spaced apart resistance segments and interconnecting apexes. The plurality of resistance segments and apexes are located in a single plane and are interconnected between the second resistance segment and the second terminal such that a continuous serpentine resistance element extends from the first terminal to the second terminal at the center of the resistor.
One feature of a resistor in accord with the principles of the present invention is that for a pair of resistance segments, the length of the resistance segment located nearer the center of the resistor is shorter than the length of resistance segment located further from the center of the resistor.
In accordance with a yet more limited aspect of the present invention, the first and second resistance elements are generally circular.
In accordance with another aspect of the present invention, the distance between adjacent concentric resistance sections at the perimeter of the resistor body is greater than the distance between adjacent concentric resistance sections near the center of the resistor.
Yet another aspect of the present invention, the apexes joining the adjacent concentric resistance sections lie adjacent a radial line extending from the center of the resistor.
In accordance with another aspect of the present invention, the single plane serpentine resistor is electrically connected to the corona ring and the second terminal. The plurality of concentric adjacent spaced apart resistance segments are connected by apexes. The value of the difference in electrical potential between adjacent apexes near the center of the resistor is less than the value of the difference in electrical potential between adjacent apexes near the perimeter of the resistor. In accordance with a more limited aspect of the invention, the change in value of the difference in electrical potential between adjacent apexes is non-linear when sequentially comparing the voltage difference between apexes from the resistor perimeter to the voltage difference between apexes near the resistor center.
In accordance with another aspect of the invention, an apparatus for an x-ray tube utilizes the resistor of the present invention. The x-ray tube includes a cathode assembly, an anode assembly, a bearing assembly rotatably supporting the anode assembly, and an envelope enclosing the anode assembly, the bearing assembly and the cathode assembly in a vacuum. A low inductance resistor is included which has a body. A first terminal is located away from the center of the resistor. A corona ring within the body is electrically connected to the first terminal. A second terminal is located at the center of the resistor and is electrically connected to the bearing assembly. The low inductance resistor includes a serpentine resistance element having a first end and a second end. The first end is electrically connected to the corona ring. The resistance element extends in a plurality of adjacent concentric resistance sections located in a single plane. The adjacent resistance sections reverse direction at apexes joining the adjacent concentric resistance sections. Each adjacent concentric resistance section carrying electric current in an opposite direction than the electric current in the next adjacent concentric resistance section. The second end is electrically connected to the second terminal.
The present invention provides the foregoing and other features hereinafter described and particularly pointed out in the claims. The following description and accompanying drawings set forth certain illustrative embodiments of the invention. It is to be appreciated that different embodiments of the invention may take form in various components and arrangements of components. These described embodiments being indicative of but a few of the various ways in which the principles of the invention may be employed. The drawings are only for the purpose of illustrating a preferred embodiment and are not to be construed as limiting the invention.